Oxygen Permeation Behaviors and Hardening Effect of Titanium Alloys at High Temperature

In order to improve the surface hardness and wear resistance of Ti and Ti alloy components, an oxygen permeation treatment (OPT) was developed. The oxygen permeation behaviors of three Ti alloys, TA2, TB5 and TC11, treated in air with O-P medium at high temperature have been studied. The results show that the 0-P treatment can significantly improve the surface hardness of Ti alloys. The oxidation mass-gain of β-Ti alloy (TB5) is much higher than α-Ti alloy (TA2) under the same condition, while α+β Ti alloy (TC11) is the lowest. All the Ti alloys treated at this condition produce two surface layers: the outer layer consists mainly of TiO2, as well as trace of other oxides, and the inner layer consists of a Ti-O interstitial solid solution formed by the diffusion of oxygen in α crystal lattice. Thick scales of β Ti alloy (TB5) are easily formed depending mainly on the poor solid solution content of oxygen, while deep solution layer can be formed since partial β phase has been transformed into α phase. The scales of α-β Ti alloy (TC11) are very thin and compact. Aluminum-rich zone, as well as deficient zone, is found in oxide layers. A crystallographic characterization of oxygen solution layer has been performed and evaluated by crystallographic lattice constant.

Oxygen permeation and phase structure properties of partially A-site substituted BaCo_(0.7)Fe_(0.225)Ta_(0.075)O_(3-δ) perovskites

Ba0.9R0.1Co0.TFe0.225Ta0.07503-δ （BRCFT, R = Ca, La or Sr） membranes were synthesized by a solid-state reaction. Metal cation Ca2＋, La3＋ or Sr2＋ doping on A-site partially substituted Ba2＋ in BaCoo.TFe0.225Ta0.07503-δ oxides, and its subsequent effects on phase structure stability, oxygen permeability and oxygen desorption were systematically investigated by XRD, TG-DSC, Hz-TPR, O2-TPD techniques and oxygen permeation experiments. The partial substitution with Ca2＋, La3＋ or Sr2＋, whose ionic radii are smaller than that of Ba2＋, succeeded in stabilizing the cubic perovskite structure without formation of impurity phases, as revealed by XRD analysis. Oxygen-involving experi- ments showed that BRCFT with A-site fully occupied by Ba2＋ exhibited good oxygen permeation flux under He flow, reaching about 2.3 mL.min-l .cm-2 at 900 with I mm thickness. Of all the membranes, BLCFT membrane showed better chemical stability in CO2, owing to the reduction in alkalinity of the mixed conductor oxide by La doping. In addition, we also found the stability of the perovskite structure under reducing atmospheres was strengthened by increasing the size of A-site cation （Ba2＋〉La3＋〉SrZ＋〉Ca2＋）.

Preparation and oxygen permeation properties of SrFe(Cu)O_(3-δ) dense ceramic membranes

Mixed oxygen-ionic and electronic conducting membranes of SrFe（Cu）O3-δ were prepared by solid-state reaction method. The crystal structure, oxygen nonstoichiometry, and phase stability of the materials were studied by TGA and XRD. Oxygen permeation fluxes through these membranes were studied at operating temperature ranging from 750 to 950 ℃. Results showed that doping Cu in SrFeO3-δ compound had a significant effect on the formation of single-phased perovskite structure. For SrFe1-xCu2O3-δ series materials, the oxygen nonstoichiometry and the oxygen permeation flux increased considerably with the increase of Cu-doping content （x = 0.1-0.3）. The sintering property of the membrane decreased significantly when the Cu substitution amount reached 40%. SrFe0.7Cu0.3O3-δ showed high oxygen permeation flux, but SrCuO2 and Sr2Fe2O5 phases formed in the compound after oxygen permeation test induced cracks in the membrane.

Electrifying Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ) for focalized heating in oxygen transport membranes

Industry decarbonization requires the development of highly efficient and flexible technologies relying on renewable energy resources,especially biomass and solar/wind electricity.In the case of pure oxygen production,oxygen transport membranes(OTMs)appear as an alternative technology for the cryogenic distillation of air,the industrially-established process of producing oxygen.Moreover,OTMs could provide oxygen from different sources(air,water,CO_(2),etc.),and they are more flexible in adapting to current processes,producing oxygen at 700^(-1)000℃.Furthermore,OTMs can be integrated into catalytic membrane reactors,providing new pathways for different processes.The first part of this study was focused on electrification on a traditional OTM material(Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ)),imposing different electric currents/voltages along a capillary membrane.Thanks to the emerging Joule effect,the membrane-surface temperature and the associated O_(2) permeation flux could be adjusted.Here,the OTM is electrically and locally heated and reaches 900℃on the surface,whereas the surrounding of the membrane was maintained at 650℃.The O_(2)permeation flux reached for the electrified membranes was~3.7 NmL min^(-1)cm^(-2),corresponding to the flux obtained with an OTM non-electrified at 900℃.The influence of depositing a porous Ce_(0.8)Tb_(0.2)O_(2-δ) catalytic/protective layer on the outer membrane surface revealed that lower surface temperatures(830℃)were detected at the same imposed electric power.Finally,the electrification concept was demonstrated in a catalytic membrane reactor(CMR)where the oxidative dehydrogenation of ethane(ODHE)was carried out.ODHE reaction is very sensitive to temperature,and here,we demonstrate an improvement of the ethylene yield by reaching moderate temperatures in the reaction chamber while the O_(2) injection into the reaction can be easily fine-tuned.

Oxygen Permeation and Stability of Ce_(0.8)Gd_(0.2)O_(2-δ)-PrBaCo_(2-x)FexO_(5+δ) Dual-phase Composite Membranes

Dual-phase membranes of 60 wt% Ce0.8Gd0.2O2-δ-40 wt% Pr Ba Co2exFexO3 d（0 x 2） were prepared by a combined citrate and ethylene diamine tetraacetic acid（EDTA） complexing method. X-ray diffraction（XRD）results revealed the good chemical compatibility between ion-conducting phase CGO and electron-conducting phases PBC2 xFxO after sintering in air. The Fe ionic dopant had a significant effect on the phase structure stability and oxygen permeability under CO2 atmosphere, which was confirmed by XRD, thermogravimetrye differential scanning calorimetry（TGeD SC）, scanning electron microscopy（SEM） and oxygen permeation experiments. CGOeP BC0.5F1.5O dual-phase membrane demonstrated a stable oxygen permeation flux of2.71x10-7mol cm 2s 1with 50 mol% He/CO2 as the sweep gas at 925 C, and this value was much higher than that of perovskite-type membranes. The rate-limiting step in the oxygen permeation process changed from the bulk diffusion to the surface oxygen exchange when the CGOeP BC0.5F1.5O membrane thickness decreased to 0.8 mm or less. Due to the high oxygen permeation fluxes and the excellent structural stability under CO2 atmosphere, the CGOeP BC0.5F1.5O membrane is a great potential candidate material for separating oxygen from air in the oxy-fuel combustion techniques.

Enhanced Oxidative Coupling of Methane with Oxygen Permeation Membrane

Oxidative coupling of methane to ethylene is of high importance to the future of light olefin industry.However,the carbon atom efficiency is normally below 50%in gas phase reaction which is limited to the overoxidation of methane to carbon dioxide with oxidants.Here we present an alternative approach of electrochemical oxidation of methane in an oxygen permeation membrane reactor and show the highest conversion of methane and C2 selectivity of 28%and 40.2%at 1150℃,respectively.We prepare the 100-μm-thick perovskite(La0.8 Sr0.2)1-x Cr0.5 Fe0.5 O3-δ(LSCrF)dense membrane(La0.8 Sr0.2)1-xCr0.5 Fe0.5O3-δ(LSCrF–Fe)(x=0,0.02,0.05 and 0.10)scaffolds while the excess of Fe would be exsolved on porous skeleton to create metal-oxide interfaces toward methane oxidation.The metal-oxide interfaces not only facilitate the activation of C–H bond in methane but also enhance the coking resistance.

Preparation and characterization of Ce_(0.8)Sm_(0.2)O_(1.9)-La_(0.8)Sr_(0.2)Cr_(0.5)Fe_(0.5)O_(3-δ) dual-phase membranes for oxygen permeation

In this study,with La_(0.8)Sr_(0.2)Cr_(0.5)Fe_(0.5)O_(3-δ)(LSCrF)and Ce_(0.8)Sm_(0.2)O_(1.9)(SDC)taken as the electronic conducting phase and the oxide ion conductor,respectively,dual-phase composite ceramic membranes with or without 3%(in mole)Y_(2)O_(3) stable ZrO_(2)(3YSZ)support were prepared and tested as oxygen separation membranes.Then,the oxygen transmission performance was tested by the electrochemical method based on the limiting current.The results obtained with this method coincide with those acquired by means of gas chromatography.Furthermore,the dependences of oxygen permeation rate on oxygen partial pressure,temperature and ceramic membrane thickness were analyzed.When air was used as the sweeping gas on the porous 3YSZ supported SDC-LSCrF dual-phase membrane,the oxygen permeation rate can reach 0.73,0.82,0.90 and 0.97 mL cm^(-2) min^(-1) at 650,700,750 and 800℃,respectively.

Perovskite-type oxygen-permeable membrane BaCo_(0.7)Fe_(0.2)Nb_(0.1)O_(3-δ) for partial oxidation of methane in coke oven gas to hydrogen

Perovskite-type oxygen-permeable membrane reactors of BaCo0.7Fe0.2Nb0.1O3-δ （BCFNO） packed with Ru-based catalyst had high oxygen permeability and could be used for hydrogen production by partial oxidation of methane in coke oven gas （COG）. At 1173 K, 94% of methane conversion, 85% of H2 selectivity, 107% of CO selectivity, and as high as 15.4 mL·cm^-2·min^-1 of oxygen permeation flux were obtained. The BCFNO membrane itself had poor catalytic activity to partial oxidation of CH4 in COG. During continuous operation for 70 h at 1173 K, no degradation of the membrane reaction performance was observed. XRD and SEM characterization also demonstrated that the BCFNO membrane reactor exhibited good stability in partial oxidation of methane in COG.

Syngas Production Using an Oxygen-Permeating Membrane Reactor with Cofeed of Methane and Carbon Dioxide

CH4-CO2-O-2 reforming to syngas in a never Ba0.5Sr0.5Co0.8Fe0.2O3.delta oxygen-permeable membrane reactor using LiLaNiO/gamma-Al2O3 as catalyst was successfully reported. Excellent reaction performance was achieved with around 92% methane conversion efficiency, 95% CO2 conversion rate, and nearly 8.5mL/min.cm(2) oxygen permeation flux. In contrast to the oxygen permeation model with the presence of large concentration of CO2 (under such condition the oxygen permeation flux deteriorates with time), the oxygen permeation flux is really stable under the CH4CO2-O-2 reforming condition.

Hydrogen production from coke oven gas over LiNi/γ-Al_2O_3 catalyst modified by rare earth metal oxide in a membrane reactor

The performance of LiNi/γ-Al2O3 catalysts modified by rare earth metal oxide （La2O3 or CeO2） packed on BCFNO membrane reactor was discussed for the partial oxidation of methane （POM） in coke oven gas （COG） at 875 ℃. The NiO/γ-Al2O3 catalysts with different amounts of La2O3 and CeO2 were prepared with the same preparation method and under the same condition in order to compare the reaction performance （oxygen permeation, CH4 conversion, H2 and CO selectivity） on the membrane reactor. The results show that the oxygen permeation flux increased significantly with LiNiREOx/γ-Al2O3 （RE = La or Ce） catalysts by adding the element of rare earth especially the Ce during the POM in COG. Such as, the Li15wt%CeO29wt%NiO/γ-Al2O3 catalyst with an oxygen permeation flux of 24.71 ml·cm^-2·min^-1 and a high CH4 conversion was obtained in 875 ℃. The resulted high oxygen permeation flux may be due to the added Ce that inhibited the strong interaction between Ni and Al2O3 to form the NiAl2O4 phase. In addition, the introduction of Ce leads up to an important property of storing and releasing oxygen.

Microstructure evolution and oxidation states of Co in perovskite-type oxide Ba_(1.0)Co_(0.7)Fe_(0.2)Nb_(0.1)O_(3-δ) annealed in CO_2 atmosphere

Ba1.0Co0.7Fe0.2Nb0.1O3-γ（BCFN） oxide with perovskite cubic structure was synthesized by solid state reaction method. COa corrosion of BCFN membrane was investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, diffuse reflectance infrared Fourier- transformed spectroscopy （DRIFT） and X-ray absorption fine structure spectroscopy （XAFS）. Cobalt （Co） K-edge absorption spectra of BCFN annealed in COa reveal that the oxidation states of Co in all the samples were larger than ＋3 and they decreased with the increase of calcination time. At 800 ℃, 1% CO2 introduced into He could speed up the reduction of Co cations in comparison with pure He. In addition, sulfate ions in the bulk of BCFN membrane preferred to migrate to the surface under CO2 calcination and form monoclinic Ba（CO3）0.9（SO4）0.1 besides orthorhombic witherite. Moreover, SEM results indicate that the nucleation and growth of carbonates grains started at the grain boundary of the membrane.

Preparation and Characterization of Single-Phase Perovskite La_(0.6)Sr_(0.4)Co_(0.8)Fe_(0.2)O_(3-δ)

Single-phase perovskite La0.6Sr0.4Co0.8Fe0.2O3-δ has been successfully prepared by using citrate-EDTA complexation method at relatively low calcination temperature. The structure and thermal decomposition process of the complex precursor have been investigated by means of differential scanning calorimetry-thermal gravimetric analysis （DSC/TGA）, X-ray diffraction （XRD）, and Fourier transform infrared spectroscopic （FT-IR） measurements. The precursor decomposed completely and started to form perovskite-type oxide above 420 ℃ according to the differential scanning calorimetry （DSC） and thermal gravimetric analysis （TGA） results. Single-phase perovskite La0.6Sr0.4Co0.8Fe0.2O3-δ obtained has been confirmed from the XRD pattern, and no peak of SrCO3 was found by XRD of the oxides synthesized at a relatively low temperature of 800℃. The reducibility of La0.6Sr0.4Co0.8Fe0.2O3-δ was also characterized by the temperature programmed reduction （TPR） technique. Disk shaped dense La0.6Sr0.4Co0.8Fe0.2O3-δ membrane was prepared by the isostatical pressing method. The oxygen flux rate of dense La0.6Sr0.4Co0.8Fe0.2O3-δ membrane was （2.8-18）× 10^-8 mol/（cm^2.s） in the temperature range of 800-1000 ℃.

A CO and CO2 tolerating (La0.9Ca0.1)2(Ni0.75Cu0.25)O4+δ Ruddlesden-Popper membrane for oxygen separation

A series of novel dense mixed conducting ceramic membranes based on K2NiF4-type(La1-xCax)2(Ni0.75Cu0.25)O4+δwas successfully prepared through a sol-gel route.Their chemical compatibility,oxygen permeability,CO and CO2 tolerance,and long-term CO2 resistance regarding phase composition and crystal structure at different atmospheres were studied.The results show that higher Ca contents in the material lead to the formation of CaCO3.A constant oxygen permeation flux of about 0.63 mL·min−1·cm−2 at 1173 K through a 0.65 mm thick membrane was measured for(La0.9Ca0.1)2(Ni0.75Cu0.25)O4+δ,using either helium or pure CO2 as sweep gas.Steady oxygen fluxes with no sign of deterioration of this membrane were observed with increasing CO2 concentration.The membrane showed excellent chemical stability towards CO2 for more than 1360 h and phase stability in presence of CO for 4 h at high temperature.In addition,this membrane did not deteriorate in a high-energy CO2 plasma.The present work demonstrates that this(La0.9Ca0.1)2(Ni0.75Cu0.25)O4+δmembrane is a promising chemically robust candidate for oxygen separation applications.

Performance of Sm_(0.7)Sr_(0.3)CoO_(3-δ) dmembrane under CO_2-containing atmosphere

The permeability and stability of Sm_（0.7）Sr_（0.3）CoO_（3-δ）（SSCO） regarding the special requirements for carbon capture and storage（CCS） application were investigated.Pure CO_ was used as the sweep gas at 900 °C,leading to that the oxygen permeation flux decreases by about 34 %.Several cycles of changing the sweep gas between helium and CO_2 indicate the good reversibility of this degradation.Both carbonate formation and adsorption of CO_2 on the membrane surface are responsible for the degradation of the membrane performance.The better CO_2 resistance results from the substitution of Sm for Sr due to the higher acidity of Sm_2O_3（1.278） than that of Sr O（0.978） and a discontinuous layer of carbonate.